How about Rosanna Arquette in The Whole Nine Yards ? She was meant to be French Canadian, and just sounded like a mentally challenged person with a speech impediment. And spittle problems.

I have to argue with two of these:

Pete Postlethwaite in The Usual Suspects - I don't think he was meant to have any accent in particular. He's English, but I think he was meant to be generically foreign, and we weren't really supposed to know where he was from.

Brad Pitt in Snatch - by most knowledgable accounts I've heard, he did a bang-up job with the Pikey accent.

And Rob Schneider in 50 First Dates ? - Surely we weren't meant to think he was trying to do a passable accent for any recognizable ethnicity.

I included people on here whose accents I liked just to have them included for the people who may not have liked them, for example I like Norman Reedus and Sean Patrick Flannery's accents in The Boondock Saints and Brad Pitt in Snatch. The reason Brad Pitt spoke with the accent he did in snatch is because he could not do a British accent.

I also added people from movies that I haven't seen, such as The Usual Suspects, on recommendations from friends. Since I haven't seen The Usual Suspects, I don't know anything about Pete Postlethwaite's accent.

And this is not meant to be a poll on accents of recognizable ethnicities but accents in general.

Ah, me understand good. However, I think it's a lot easier to judge if someone's done a bad accent if they're trying to do something that's supposed to be recognizable. Dick Van Dyke's cockney was bloody abysmal, but if he'd been trying to do something random and vague, then he wouldn't have missed the mark so badly, because the mark wouldn't have been fixed.

Also, where did you hear that Brad Pitt did Pikey because he couldn't do English? I'm honestly curious - I thought his being Pikey was pretty integral to the character and the environment the character lived in. Having him do the Queen's English would have been seriously out of place. Of course, I suppose he could have been Scouser and still had the same ill-brought up effect. :) (I kid! I kid because I love.)

Brad Pitt, who was a big fan of Lock, Stock and Two Smoking Barrels (1998), approached director Guy Ritchie and asked for a role in Snatch. When Ritchie found Pitt couldn't master a London accent, he gave him the role of Mickey the Gypsy.http://www.imdb.com/title/tt0208092/trivia

Also I believe that Guy Ritchie mentions it on the Audio commentary on the DVD.

Sorry to drag this out, but I thought Oedipus meant Ritchie considered casting Pitt as one of the other roles in the movie. And when that didn't work out, he cast Pitt as the boxer, who was always meant to be Pikey.